Sensitized blue print paper and coating therefor



Patented Aug. 9, 1938 UNITED STATES PATENT OFFICE SENSITIZED BLUE PRINT PAPER AND COAT- ING THEREFOR Alger P. Reynolds, North Beverly, Masa, assignor to Spaulding-Moss Company, Boston, Mass, corporation of Massachusetts No Drawing. Application August 21, 1937, Serial No. 160,369

1 Claim.

The present invention relates to the art of making blue prints by coating a suitable base .(which base for-the sake of convenience is hereinafter referred to as "paper, but which ex-' developing bath apermanent blue color is obtained wherever the light reached the sensitized paper during exposure.

The primary object of my invention is to pro vide a more highly sensitive coating for blue print paper which requires exposure to light a shorter period of time during printing, thereby providing a faster paper.

Another object is to provide such a coating which results in a deeper blue background in the exposed areas in the final print.

A further object is to eliminate fogging of the white unexposed surfaces or lines in the final print.

Another objectis to provide such an improved 2 sensitized paper which may be kept for longer periods .of time in sensitized condition without deterioration prior to exposure to light.

A further object is to provide a coating solution which may be applied to the paper at room tem- 30 perature, i. e., without heating the solution during coating.

Other objects are to provide an improved blue print paper coating and an improved coating solution.

5 In theprior art it has long been the established practice to sensitize the paper with a coating solution having as its essential elements a water solution of a light-sensitive iron saltin the ferric translucent paper (or other suitable medium) is' transposed over the sensitized surface of the blue print paper and'is then exposed. to light for a period of time suflicient to insure reduction of the light-sensitive ferric salt to a ferrous salt. Upon washing the exposed paper with water the ferrous salt combines with the ferricyanide to form an insoluble blue compound, and the unexposed and unreducedferric salt which is found on the blue print paper beneath the opaque lines of the diawhen one or more of the class comprising alkaline gram is dissolved out of the blue print paper leaving a white diagram upon a blue background. After being washed with water the exposed paper is commonly washed in a bath containing an oxidizing agent, such as potassium bichromate, thereby bringing out in full the blue background and the paper is then again washed with water and subsequently dried.

Either partial or complete substitution of a ferrocyanide for the ferricyanide in the above-re- 1o ferred to prior art coating bath has been suggested to thereby obtain a faster printing paper which givessharper white lines upon a deeper blue background. These suggestions will be found in the British patent to Holden No. 418,369 of 15 1934. In the only suggestions .of the use of ferrocyanide found in the prior art, it has been stated that the entire coating solution, 1. e.. water, light-sensitive ferric salt and ferrocyanide (and ferricyanide, if the latter is also used) must not 20 be acid, and this coating solution as a whole has been made" either neutral or alkaline by the use in the solution of suitable combinations of borax, ammonia, sodium hydroxide or potassium oxalate,

- and the use of these chemicals has been understood by the art to be buffering" the coating solu-. tion to neutral or above, i. e., neutral or alkaline.

when an alkali metal ferrocyanide, such as potassium, sodium or ammonium ferrocyanide, is added to a water solution of a light-sensitive ferric salt, such as ferric ammonium oxalate, an insoluble blue substance is formed before exposure to light and after exposure and washing a paper coated with such a coating solution retains a considerable amount of the blue substance upon the unexposed areas, so that these unexposed areas are fogged with blue rather than remaining a clear white. Thus if a ferrocyanide is to be used in a coating solution something must be present which prevents the retention of this blue substance upon the unexposed areas thus permitting it to be washed completely off after printing, leaving clear whites. The prior art has believed that the incorporation of any suitable "buffer" in this ferrocyanide coating solution to thereby make the solution as a whole neutral or alkaline would prevent this fogging of the whites.

In my copending application Serial No. 65,661, filed February 25, 1936, I have pointed out that hydrolyzable alkali metal compoundsare used in a ferrocyanide coating in such proportions that whites is eliminated, the paper prints faster, with sharper white lines upon a deeper blue background and the coated paper is easier to keep in sensitized condition.

The group of chemicals known as the alkali metal phosphates includes some phosphates which are acid hydrolyzable in a water solution and some phosphates which are alkaline hydrolyzable in a water solution. I have discovered that the acid hydrolyzable alkali metal phosphates may be incorporated in a coating solution containing a light-sensitive iron salt in the ferric state, and an alkali metal ferrocyanide to give as improved, if not a better coating than is obtained by the use of alkaline hydrolyzable stances which function per se in the coating to prevent the formation of an insoluble blue compound prior to exposure to light. Thus the alkali metal phosphates as a class are particularly adaptable for use in such a ferrocyanide blue print coating since both those which are acid hydrolyzable and those which are alkaline hydrolyzable are fully operative to produce the desired result.

By the term "alkali metal" in this specification and the annexed claim, I intend to include only potassium, sodium, cesium, rubidium, lithium, and ammonia, which latter substance although it is not, strictly speaking, a metal, is commonly classed in the alkali metal group. By alkaline hydrolyzable compounds wherever said term is used in this specification and the annexed claim,

I intend to include only water soluble salts or compounds, the water solutions of which have a pH value of greater than I. By acid hydrolyzable" compounds wherever said term is used in this specification and the annexed claim, I intend to include only water soluble salts or compounds, the water solutions of which have a pH value of less than 7.

Thus the class of chemicals referred to as alkali metal. phosphates" includes suchchemicals as:

a. Monosodium phosphate, monopotassium phosphate and monoammonium phosphate.

b. Sodium pyrophosphate, trlsodium phosphate and disodium phosphate.

c. Sodium borophosphate, sodium glycerophosphate, ammonium sodium phosphate and magnesium ammonium phosphate.

The examples set forth in'paragraph a are "acid hydrolyzable alkali metal phosphates", i. e.

each hydrolyzes acid in a water solution and the examples set forth in paragraphs 12 and c are "alkaline hydrolynble alkali metal phosphates. The examples set forth in paragraph c are socalled complex's.lkai i metal phosphates, i. e. alkali metal phosphates in which one or more alkali metals and one or more phosphate radicals are present. I intend toinciude such complex alkali metal phosphates within the term alkali metal phosphates" even if such phosphaies contain in addition metals other than alkali metals or acid radicals, other than phosphate radicals;

Other members of thealkali metal phosphate classwillbeapparenttopersonsskilledintheart.

An example of a' costing solution using an acid hydrolyzable alkali metal phosphate is as follows:

This solution is of a deep blue color andhas a pH value of 3.97. If the i'erricyanide is replaced in whole or in part by ferrocyanide the amount of monopotasslum phosphate should ordinarily be increased.

Examples of coating solutions using alkaline hydrolyzable alkali metal phosphates are as fol- IOWSI 6000 grams ferric annnonium oxalate and 900 grams sodium pyrophosphate, dissolved in 15,000 cc. of water, and to this add: -900 grams potassium ferrocyanide and 750 grams potassium ferricyanide dissolved in 15,000 cc. of water.

This solution is of a deep blue color and has a V pH value of 8.19.

6000 grams ferris sodium oxalate and 2485 grams sodium glyoerophosphate dissolved in 15,000 cc. of water, and to this add: 422 grams of potassium ferrocyanide and 329 grams of potassium ferricyanide dissolved in 15,000 cc. of water.

This solution is of a deep blue color and has a pH value of 0.03. I

If the ferricyanide is replaced in whole or in part by ferrocyanide in either of the foregoing solutions the respective amounts of sodium pyrophosphate and sodium glycerophosphate should ordinarily be increased.

It will be apparent to one skilled in the art that if alkali metal ions are present in a coating solution containing a light-sensitive ferric salt and a ferrocyanide, the incorporation of phosphates or any phosphoric acid into such 'a solution to thereby form an alkali metal phosphate yould be within the scope of this invention asequivalent to use ab inltio of an alkali metal phosphate.

It will be apparent to persons skilled in this art that it is within the scope of this invention and the annexed claim to first coat the paper with a solution containing, or to prepare a paper with a sizing containing, one or more alkali metal phosphates and then to subsequently coat this paper with a solution containing a light-sensitive ferric salt and a ferrocyanide, since the two coatings would combine to form the coating of this invention.

In this specification and the annexed claim when I refer to a coating containing alkali metal phosphates. light-sensitive iron salts in the ferric state, and ferrocyanides, I intend to include a coating solution containing, a coating containing, and a paper coated with a coating containing the reaction products of said alkali metal phosphates, said light-sensitive ferric salts and said ferrocyanides because I do not know whether or not said chemicals retain their original identity after they are brought together in a coating solution It will be further understood that chemicals other than alkali metal phosphates, light-sensltive ferric salts, ierrocyanides, and ferricyanides may be incorporated in the coating solution or in the coating on the paper without departing from. my invention. So long as the water solution of the coating is acid and contains at least one alkali metal phosphate, a light-sensitive ferric salt, and an alkali metal ferrocyanide, it is intended to be within the scope of the annexed claim, even if the coating solution or the coating on the paper contains other chemicals in addition to those specified.

Suitable light-sensitive iron salts in the ferric state other than ferric ammonium or ferric sodium oxalate, suitable alkali metal ierrocyanides other than potassium ferrocyanide, and suitable ierricyanides other than potassium ferricyanide will be apparent to those skilled in the art.

The determination of the alkalinity, neutrality, or acidity of hydrolyzation of the compounds and salts recited in this specification and the determination of the pH values of coating solutions given in this specification has been made by utilizing a potentiometer and a glass electrode, which method is well known to those skilled in the art. By an acid coating solution I mean one which has a pH value of less than 7 and which value is determined by said method orby one which is equivalently accurate. By an alkaline hydrolyzable compound or salt I mean one the water solution of which has a pH value greater than '7 and which value is accurately determined by said method or its equivalent. By an acid hydrolyzable compound or salt I mean one the water solution of which has a pH value of less than 7 and which value is ace cnrately determined by said a method or its equivalent- In use the paper is sensitized by coating at least one surface thereof with a coating solution made in accordance with this invention, and it is then dried and ready for use. Such a coated paper is dark blue-green in color as distinguished from the lemon-yellow color of prior art paper. The coating may be effected by brushing, by spraying, by passing the paper through a machine in contact with rollers which dip into a bath or the solution or by any other suitable coating procedure. The diagram is then superimposed upon the sensitized surface of the dry paper, is printed by exposure to light, and the printed or exposed paper is passedthrough a water bath after which it may be hung up to dry. If desired. before this iinal drying the paper may be passed through an oxidizing bath, then through another water bath and then dried.

The acidity of the coating solution as a whole increases the speed of reduction of the lightsensitive iron salt and 131113 an acid coating solu- 'tion provides a much faster printing paper than a coating solution which is neutral or alkaline.

Paper coated with theacid coating solutions of a light sensitive ferric salt and ferrocyanide made in accordance with my invention may be kept in sensitized condition without spoilage for much longer periods of time before exposure than the papers of the prior art, and will produce perfectly clear and unfogged whites when subsequently printed. The acidity oi my coating solution causes the size in the paper to swell and thus to absorb the coating solution much more readily than neutral or alkaline solutions, and consequently a less concentrated coating solution is required to provide suficient of the sensitized coating upon the surface of the paper for a satisfactory deep blue background in the final print, whereas in the use of neutral or alkaline solutions it is necessary to use solutions which are highly concentrated andwhich require heating to keep the chemicals in solution before and during coating.

It is to be understood that my invention is not dependent upon the theories which I have set iorth as descriptive of the chemical actions involved nor dependent upon the soundness or accuracy oi theoretical statements so, advanced, such theories or theoretical statements are merely what I believe to be correct and I intend to claim everything inherent in my invention which is novel in view of the prior art.

I claim:

A sensitized blue print paper coating containing an acid hydrolyzable alkali metal phosphate, a light-sensitive iron salt in the ferric state and an alkali metal ierrocyanide, the proportions of said alkali metal phosphate to the other ingredients oi the coating being such that a water solution of said coating has a pH value of less than i but not sufficiently low to permit the formation of an insoluble blue compound prior to exposure to light.

AJLEER P. REYNOLDS. 

